I recently made an unexpected finding that the cerebellum of the frog tadpole is so immature that the external granular layer is altogether absent and that, in the case of the bullfrog tadpole, it remains in this condition for 2 or 3 years and undergoes maturation at the time of metamorphosis. I also found that the tadpole cerebellum can be induced to undergo precocious maturation by thyroid treatment. The present proposal is designed to undertake a detailed study of neurogenesis, morphogenesis and synaptogenesis in the cerebellum of the frog tadpole, with particular reference to the maturation processes that take place during both normal and thyroid-induced metamorphosis of the tadpole into the froglet. In this grant period the study will be mainly descriptive of normal developmental changes. The experimental methods used will include Golgi silver impregnation, electron microscopy and radioautography.